As you know: Time is money and if the casting directors are able to safe time and not bringing the wrong actor to the casting - it helps to watch first a demo reel / actor reel before they invite you for a casting.
On a demo reel / actor reel the casting director can see if you are good at acting, They also can hear your voice.
If you have the right voice is important, too. With an actor reel / demo reel they get to see how you look. A performance on your demo reel / actor reel get a feeling if you are likable and if you are a fit for the role and character they are looking for.